Fuel cells designed for use in portable applications, such as in vehicles, are advantageously of low weight, small volume and safe in all orientations (e.g., upside down). For these reasons, the proton exchange membrane (PEM) fuel cell has been widely developed for use in vehicles. Various forms of degradation have been noted; one form is flooding due to water migration to and accumulation at the cathodes when the fuel cell, at rest between uses, is cooled to very low temperatures, including below the freezing temperature of water. However, various methodologies have been proposed to overcome the water migration problem, such as draining and purging, to eliminate water before the fuel cell has time to cool.
On the other hand, there are degradations which have not had preventative or curative measures devised for them. One such fuel cell stack degradation involves the erosion of membranes as a consequence of interaction with contaminants which accelerate degradation. Although the performance degradation may hardly be noticed up to a point, perforation of the membrane does occur with crossover of reactant gas, typically hydrogen crossing to the cathode and mixing with oxygen. Combustion creates a hot spot which in turn may cause perforation of adjacent membranes, thereby participating in a cascading effect, with several or many fuel cells exhibiting crossover combustion. Several failed cells cause significant reduction in performance (voltage versus current density) as well as reduction in capacity (maximum load). Such perforations ultimately result in the need to completely rebuild or replace the fuel cell stack.
One known degradation accelerant is silica, which is nearly impossible to eliminate from the entire surface of both sides of all of the membranes in a stack. The silica reacts with the catalyst adjacent to the membrane to form silicates, thus eroding a portion of the membrane. Other contaminants which have the potential for causing excessive membrane degradation rates and possible perforation of the membrane include sulfates and chlorates.
A problem with several types of fuel cells, including phosphoric acid fuel cells and molten carbonate fuel cells, as well as PEM fuel cells, is the cost and the weight of the axial loading system. The bolts, nuts and the endplates are heavy and expensive.